Add median flux slider

[jernestmyers]

Implements the following UI elements in the AllSkyMap on the main page:

1. Details about the number of sources displayed in the top middle, plus a "Filter by median flux" button in the top right.

2. A filter menu when the filter button is clicked

3. When filters are applied, the details about the number of sources (top middle) updates to include the applied filter with a button to remove the filter

[jernestmyers]

Note that I mapped socolors to the filter's applied frequency band. When unselected, we use a fallback color defined in socolors.

Screenshot showing unfiltered, unselected frequency band:

Screenshot showing filtering on f090: